1. Field of the Invention
This invention relates to a toner for developing an electrostatic image, and more particularly to a toner for developing an elctrostatic image formed by electrophotography.
2. Related Background Art
Many electrophotographic processes are known, as disclosed in U.S. Pat. No. 2,297,691, etc.
The electrophotographic process is a process which comprises forming electrostatic latent images on a photosensitive member by various means usually utilizing a photoconductive substance, developing the latent images by a toner, transferring the toner images onto a transfer member such as paper sheet, when required, and then fixing the toner images by heating, pressing or heating-pressing, thereby obtaining a copy.
For the development of latent images by a toner or for fixing toner images, various processes have been so far proposed and processes suitable for the respective image-forming processes have been employed.
Recently, high speed copying and higher image quality have been required in the electrophotographic process, and also the improvement of color miscibility of toners themselves has been in demand as a result of full colorization. Furthermore, fixation at a lower temperature, smaller particle sizes and lower melt viscosity have been required for toners.
A well known, conventional process for preparing a toner comprises melt-mixing a thermoplastic resin, a coloring agent such as a dye or pigment, and an additive such as a charge-controlling agent, uniformly dispersing the components in the mixture, then cooling the molten mixture, finely pulverizing the cooled mixture and classifying the finely pulverized product by a classifier, thereby obtaining a toner with a desired particle size.
For the toner preparation using the pulverization process, a brittleness is required for the binding resin to give a sufficient pulverizability he cooled product. Therefore, the toners prepared by the pullverization process have sharp projections on the surfaces and thus are highly susceptible to further fine pulverization or powdering in the developing unit, resulting in an increased fogged image or unwanted scattering in the machine.
For the fixation of a toner at a lower temperature, a lower melt viscosity is generally required for the resin. Use of a cross-linking agent to give a brittleness is against the fixation of the toner at a lower temperature and thus is not preferable. Furthermore, the toners prepared by the pulverization process are generally in an irregular shape and thus have a limit to faithful reproduction of latent images and are not favorable for the higher image quality. To obtain the higher image quality using toners prepared by the pulverization process, it is necessary to make further size reduction to smaller particle sizes. However, the brittleness relating to the pulverization efficiency of the binding resin is hardly consistent with the fixability and with the heat characteristics relating to the preservability. It is difficult to fully satisfy these properties at the same time.
Other than the toners with irregular shapes prepared by the pulverization process, Japanese Patent Publication No. 56-13945 proposes a process for obtaining a spherical toner by melt spray; Japanese Patent Publication No.57-51676 proposes a process for obtaining a spherical toner by adding a small amount of an organic solvent to toners with irregular shape, followed by stirring under cooling; and Japanese Patent Publications 36-10231 and Japanese Patent Application Laid Open Nos. 59-53856, 59-61842, etc. propose processes for obtaining a spherical toner by suspension polymerization. With these spherical toners having uniform shapes, latent images, particularly edges of the latent images, can be faithfully developed. That is, these spherical toners are suitable for higher image quality. In the case of spherical toners prepared by the polymerization process, reduction of particles to smaller particle sizes can be readily carried out, and thus the polymerization process is more suitable for higher image quality.
Japanese patent Applications Laid-Open Nos. 59-53856, 59-61842, etc. propose processes for obtaining spherical toners containing a release agent by a polymerization process. According to the process, a monomer system is made into particles in water under a high shearing force and thus fine particles can be readily formed, and the nonpolar release agent is included in he particles. Furthermore, a broad allowance can be obtained for the amount of the release agent to be added, because of absence of the pulverization step. Still furthermore, the release agent melts at the hot roll fixation to show a release effect and acts as a good heat couductor, when melted. accelerates the melting rate of the binding resin. Fixation of a toner at a lower temperature and an offset prevention effect can be obtained thereby.
In the case of using resin particles as toners, on the other hand, the toners generally contain various additives as characteristic-endowing agents. For example, a flowability-endowing agent is added to the toners to increase the flowability of toners, or charge-controlling particles are added to the toners to prevent the charge-up of toners.
However, in the case of spherical toners having no breaks, the characteristics are readily deteriorated when mixed with various additives and thus it is hard to obtain a toner with less susceptibility to deterioration and sufficient durability.
With recent full colorization of electrophotographic images, miscibility of at least three colors and fixability of at least three toner layers have been important problems. Japanese Patent Application Laid-Open No. 63-301960 proposes a color toner using polyester resin as a binding resin. The color toner has a considerably high level of color miscibility and fixability, but a further improvement of image quality is required.
In the case of toners prepared by the pulverization process, in which further size reduction is difficult because of the pulverization efficiency in the toner production process and heat characteristics of the toner, it has been required to overcome the poor image quality due to the irregular shapes of the toners.